The Intergovernmental Panel on Climate Change (IPCC) also projected sea levels would rise between 26 and 82 centimetres by 2100, and warned of a higher risk for heatwaves, floods and droughts.
Environmental activists and scientists said the first volume of the panel's long-awaited review, released in Stockholm, made it clearer than ever that Man's fossil-fuel burning must be urgently curbed to limit future damage to the climate system.
In its last report in 2007, the panel had rated its conviction at 90 per cent.
UN climate chief Christiana Figueres said the report's release was "an alarm clock moment for the world".
"To steer humanity out of the high danger zone, governments must step up immediate climate action and craft an agreement in 2015 that helps to scale up" efforts to curb greenhouse gas emissions, she said.
"Those who deny the science or choose excuses over action are playing with fire," he said in a statement.
The IPCC document is the first volume in a trilogy summarising the status of global warming and its impacts.
The panel has delivered four previous assessment reports in its 25-year history.
Each edition has pounded out an ever-louder drumbeat to warn that temperatures are rising and the risk to the climate system -- in drought, floods, storms and rising seas -- is accentuating.